Are you buying for long term or what? That's the question you need to answer within yourself, if you are buying for long term then I don't think there is any problem buying Bitcoin anytime but if you are buying for short term then you need to study the market and so on to know how feasible it will be. Either way, Bitcoin can be bought anytime and there won't be any issue even if the price dumps as it will still rise which have been seen for a long while now but same can't be said for other coins.